[opt](Variant) avoid unnecessary mem for variant extracted columns (#…

…43567)

_field_name_to_index and _field_id_to_index is unnecessary for variant
subcolumns, since they use column path as identifier

be/src/olap/tablet_schema.cpp

@@ -993,9 +993,12 @@ void TabletSchema::init_from_pb(const TabletSchemaPB& schema, bool ignore_extrac
         if (column->is_variant_type()) {
             ++_num_variant_columns;
         }
+
         _cols.emplace_back(std::move(column));
-        _field_name_to_index.emplace(StringRef(_cols.back()->name()), _num_columns);
-        _field_id_to_index[_cols.back()->unique_id()] = _num_columns;
+        if (!_cols.back()->is_extracted_column()) {
+            _field_name_to_index.emplace(StringRef(_cols.back()->name()), _num_columns);
+            _field_id_to_index[_cols.back()->unique_id()] = _num_columns;
+        }
         _num_columns++;
     }
     for (auto& index_pb : schema.index()) {